The invention relates to gas separations conducted by pressure swing adsorption (PSA), and particularly to air separation to generate concentrated oxygen or to air purification to remove carbon dioxide or vapour contaminants. In particular, the present invention relates to a rotary valve gas separation system having a plurality of rotating adsorbers disposed therein for implementing a pressure swing adsorption process for separating out the gas fractions.
Four possible applications of the present invention are:
(a) Home use medical oxygen concentrators;
(b) Portable oxygen concentrators;
(c) Ultra low power oxygen concentrators, e.g. for third world medical clinics; and
(d) Manually operated oxygen concentrator or air purifier for survival life support.
Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorber that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorber from a first end to a second end of the adsorber, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A xe2x80x9clightxe2x80x9d product, depleted in the more readily adsorbed component and enriched in the less readily adsorbed component, is then delivered from the second end of the adsorber. A xe2x80x9cheavyxe2x80x9d product enriched in the more strongly adsorbed component is exhausted from the first end of the adsorber. The light product is usually the desired product to be purified, and the heavy product often a waste product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. The heavy product (enriched in nitrogen as the more readily adsorbed component) is a desired product in the example of nitrogen separation over nitrogen-selective zeolite adsorbents. Typically, the feed is admitted to the first end of an adsorber and the light product is delivered from the second end of the adsorber when the pressure in that adsorber is elevated to a higher working pressure. The heavy product is exhausted from the first end of the adsorber at a lower working pressure. In order to achieve high purity of the light product, a fraction of the light product or gas enriched in the less readily adsorbed component is recycled back to the adsorbers as xe2x80x9clight refluxxe2x80x9d gas after pressure letdown, e.g. to perform purge, pressure equalization or repressurization steps.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbers in parallel, with directional valving at each end of each adsorber to connect the adsorbers in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. The basic pressure swing adsorption process makes inefficient use of applied energy, because of irreversible expansion over the valves while switching the adsorbers between higher and lower pressures. More complex conventional pressure swing adsorption devices achieve some improvement in efficiency by use of multiple xe2x80x9clight refluxxe2x80x9d steps, both to achieve some energy recovery by pressure equalization, and also desirably to sequence the light reflux steps so that lower purity light reflux gas reenters the second end of the adsorbers first, and higher purity light reflux gas reenters the second end of the adsorbers last, so as to maintain the correct ordering of the concentration profile in the adsorbers.
The conventional method of supporting the adsorbent is also problematic. There is a need for rigid high surface area adsorbent supports that can overcome the limitations of granular adsorbent and enable much higher cycle frequencies. High surface area laminated adsorbers, with the adsorbent supported in thin sheets separated by spacers to define flow channels between adjacent sheets, formed either as stacked assemblies or as spiral rolls, have been disclosed by Keefer (U.S. Pat. Nos. 4,968,329 and 5,082,473).
U.S. Pat. No. 4,968,329 discloses related gas separation devices with valve logic means to provide large exchanges of fresh feed gas for depleted feed gas. Such large feed exchanges may be required when concentrating one component as a desired product without excessively concentrating or accumulating other components, as in concentrating oxygen from feed air containing water vapour whose excessive concentration and accumulation would deactivate the adsorbent.
Siggelin (U.S. Pat. No. 3,176,446), Mattia (U.S. Pat. No. 4,452,612), Davidson and Lywood (U.S. Pat. No. 4,758,253), Boudet et al (U.S. Pat. No. 5,133,784), and Petit et al (U.S. Pat. No. 5,441,559) disclose PSA devices using rotary adsorber configurations. Ports for multiple angularly separated adsorbers mounted on a rotor assembly sweep past fixed ports for feed admission, product delivery and pressure equalization. In this apparatus, the relative rotation of the ports provides the function of a rotary distributor valve. All of these prior art devices use multiple adsorbers operating sequentially on the same cycle, with multiport distributor rotary valves for controlling gas flows to, from and between the adsorbers.
The prior art includes numerous examples of pressure swing adsorption and vacuum swing adsorption devices with three adsorbers operating in parallel. Thus, Hay (U.S. Pat. No. 4,969,935) and Kumar et al. (U.S. Pat. No. 5,328,503) disclose vacuum adsorption systems which do not achieve continuous operation of compressors and vacuum pumps connected at all times to one of the three adsorbers. Such operation is achieved in other three adsorber examples provided by Tagawa et al. (U.S. Pat. No. 4,781,735), Hay (U.S. Pat. No. 5,246,676), and Watson et al. (U.S. Pat. No. 5,411,578), but in each of these latter examples there is some undesirable inversion of the ordering of light product withdrawal and light reflux steps so that process efficiency is compromised.
The present invention is intended to enable high frequency operation of pressure swing and vacuum swing adsorption processes, with high energy efficiency and with compact machinery of low capital cost. The invention applies in particular to air separation.
The invention provides an apparatus for PSA separation of a gas mixture containing a more readily adsorbed component and a less readily adsorbed component, with the more readily adsorbed component being preferentially adsorbed from the feed gas mixture by an adsorbent material under increase of pressure, so as to separate from the gas mixture a heavy product gas enriched in the more readily adsorbed component, and a light product gas enriched in the less readily adsorbed component and depleted in the more readily adsorbed component. The apparatus includes compression machinery cooperating with three adsorbers mounted in a rotary PSA module.
Each adsorber has a flow path contacting adsorbent material between first and second ends of the flow path. The adsorbers are mounted at equal angular spacings in an adsorber housing, which is engaged in relative rotation with first and second valve bodies to define rotary sealing faces of first and second valves adjacent respectively the first and second ends of the adsorber flow paths. In some preferred embodiments, the adsorber housing is a rotor (the xe2x80x9cadsorber rotorxe2x80x9d) which rotates while the first and second valve bodies together form the stator. In other preferred embodiments, the adsorber housing is stationary, while the first and second valve bodies achieve the valving function. Fluid transfer means are provided to provide feed gas to the first valve body, to remove exhaust gas from the first valve body, and to deliver product gas from the second valve body.
The first valve admits feed gas to the first end of the adsorbers, and exhausts heavy product gas from the first end of the adsorbers. The second valve cooperates with the adsorbers to deliver light product gas from the second end of the adsorbers, to withdraw light reflux gas from the second end of the adsorbers, and to return light reflux gas to the second end of the adsorbers. The term xe2x80x9clight refluxxe2x80x9d refers to withdrawal of light gas (enriched in the less readily adsorbed component) from the second end of the adsorbers via the second valve, followed by pressure let-down and return of that light gas to other adsorbers at a lower pressure via the second valve. The first and second valves are operated so as to define the steps of a PSA cycle performed sequentially in each of the adsorbers, while controlling the timings of flow at specified total pressure levels between the adsorbers and the compression machinery.
The PSA process of the invention establishes the PSA cycle in each adsorber, within which the total working pressure in each adsorber is cycled between a higher pressure and a lower pressure of the PSA cycle. The higher pressure is superatmospheric, and the lower pressure may conveniently either be atmospheric or subatmospheric. The PSA process also provides intermediate pressures between the higher and lower pressure. The compression machinery of the apparatus in general includes a feed gas compressor and a heavy product gas exhauster. The exhauster would be a vacuum pump when the lower pressure is subatmospheric. When the lower pressure is atmospheric, the exhauster could be an expander, or else may be replaced by throttle means to regulate countercurrent blowdown.
In the present invention, the feed compressor will typically supply feed gas for feed pressurization of the adsorbers to the first valve means. The exhauster will typically receive heavy product gas for countercurrent blowdown of the adsorbers from the first valve means.
A buffer chamber is provided to cooperate with the second valve. The buffer chamber provides the xe2x80x9clight refluxxe2x80x9d function of accepting a portion of the gas enriched in the second component as light reflux gas from an adsorber at the higher pressure and during concurrent blowdown to reduce the pressure from the higher pressure, and then returning that gas to the same adsorber to provide purge at the lower pressure and then to provide light reflux pressurization to increase the pressure from the lower pressure. The light reflux function enables production of the light product with high purity.
The present invention performs in each adsorber the sequentially repeated steps within the cycle period as follows:
(A) Feed pressurization and production. Feed gas mixture is admitted to the first end of the adsorber during a feed time interval over approximately ⅓ of the cycle period (0Txe2x88x92T/3), commencing when the pressure within the adsorber is a first intermediate pressure between the lower pressure and the higher pressure, pressurizing the adsorber to the higher pressure (step A1), and then delivering light product gas from the second end (step A2) at a light product delivery pressure which is substantially the higher pressure less minor pressure drops due to flow friction.
(B) Withdraw from the second end a first light reflux gas enriched in the second component (preferably following step A2 of light product delivery) at approximately the higher pressure during a brief time interval at or near the end of step A (T/3).
(C) Equalization to buffer chamber. While flow at the first end of the adsorber is stopped during a concurrent blowdown time interval following step B, withdraw a second light reflux gas enriched in the second component as light reflux gas from the second end of the adsorber into the buffer chamber, and depressurizing the adsorber toward a second intermediate pressure between the higher pressure and the lower pressure.
(D) Withdraw a third light reflux gas from the second end as purge flow for another adsorber, during a brief time interval at approximately the end of step C (T/2).
(E) Countercurrent blowdown and exhaust. Exhaust a flow of gas enriched in the first component from the first end of the adsorber during an exhaust time interval (T/2xe2x88x925T/6), in step E1 to depressurize the adsorber from the second intermediate pressure to the lower pressure, and then in step E2 transferring a flow of third light reflux gas from the second end of another adsorber undergoing step D to purge the adsorber at substantially the lower pressure while continuing to exhaust gas enriched in the first component as a heavy product gas.
(F) Equalization from buffer chamber. While flow at the first end of the adsorber is stopped, supply second light reflux gas from the buffer chamber to the second end of the adsorber. This increases the pressure of the adsorber from substantially the lower pressure to the second intermediate pressure.
(G) Admit a flow of first light reflux gas from the second end of another adsorber as backfill gas to increase adsorber pressure to the first intermediate pressure for the beginning of step A of the next cycle.
It will be appreciated by those skilled in the art that alternative light reflux flow patterns may be used. For example, delete steps B and G, or delay step B to follow step A rather than overlap step A so it acts as a pressure equalization step. With appropriate porting of the second valve, the apparatus of this invention may be used to implement the process steps of prior art cycles with three adsorbers, for example as prescribed in any of the above cited U.S. Pat. Nos. 4,781,735; 4,969,935; 5,246,676; 5,328,503; and 5,411,578.
The process may be controlled by varying the cycle frequency so as to achieve desired purity, recovery and flow rates of the light product gas. Alternatively, the feed flow rate and the light product flow rate may be adjusted at a given cycle frequency, so as to achieve desired light product purity. Preferably, light product flow rate is adjusted to maintain delivery pressure in a light product receiver, by simultaneously varying feed compressor drive speed and the rotational frequency of the PSA module.
In vacuum embodiments, the first intermediate pressure and second intermediate pressure are typically approximately equal to atmospheric pressure, so that the lower pressure is subatmospheric. Alternatively, the lower pressure may be atmospheric. In air purification applications, the first component is an impurity gas or vapour, the gas mixture is air containing the impurity, and the light product is purified air. In air separation applications, the first component is nitrogen, the second component is oxygen, the adsorbent material includes a nitrogen-selective zeolite, the gas mixture is air, and the light product is enriched oxygen.
In preferred embodiments of the invention, the adsorbent is supported in the form of layered adsorbent or xe2x80x9cadsorbent laminate,xe2x80x9d formed from flexible adsorbent sheets. The adsorbent sheets are thin sheets of adsorbent with a composite reinforcement, or as an inert sheet or foil coated with the adsorbent. Flow channels are established by spacers forming parallel channels between adjacent adsorbent sheets of the experimental adsorbers has been in range of 50% to 100% of the adsorbent sheet thickness. This xe2x80x9cadsorbent laminatexe2x80x9d configuration has much lower pressure drop than packed adsorbers, and avoids the fluidization problem of packed adsorbers. The adsorbent sheets are typically in the range of 100 to 175 microns thick. The sheet-laminate provides desirable compliance to accommodate stacking or rolling errors, and spacer systems provide the necessary stability against unrestrained deflections or distortions that would degrade the uniformity of the flow channels between adjacent layers of adsorbent sheet.
According to one aspect of the invention there is provided a process for pressure swing adsorption separation of a feed gas mixture containing a more readily adsorbed component and a less readily adsorbed component, with the more readily adsorbed component being preferentially adsorbed from the feed gas mixture by an adsorbent material under increase of pressure, so as to separate from the feed gas mixture a heavy product gas enriched in the more readily adsorbed component and a light product gas enriched in the less readily adsorbed component; providing for the process a cooperating set of three adsorbers within a rotor and equally spaced angularly about the axis defined by rotation of the rotor relative to a stator, and rotating the rotor so as to generate within each adsorber cyclic variations of pressure and flow at a cyclic period defined by the frequency of rotation along a flow path contacting the adsorbent material between first and second ends of the adsorber, the cyclic variations or pressure extending between a higher pressure and a lower pressure of the process; rotating the rotor so that the first ends of the adsorbers successively communicate to feed and exhaust ports provided in a first valve surface between the rotor and the stator, and the second ends of the adsorbers successively communicate to a light product port, to light reflux exit ports and to light reflux return ports provided in a second valve surface between the rotor and the stator; the process including for each of the adsorbers in turn:
(a) supplying feed gas mixture at a feed pressure through the feed port to the adsorber over a feed interval which is substantially ⅓ of the cycle period so as to pressurize the adsorber to substantially the higher pressure, and then to deliver light product gas from the light product port at substantially the higher pressure less flow frictional pressure drops;
(b) withdrawing light reflux gas enriched in the less readily adsorbed component from the light reflux exit ports, in part to depressurize that adsorber after the feed interval;
(c) withdrawing second product gas at an exhaust pressure through the exhaust port from the adsorber over an exhaust interval which is substantially ⅓ of the cycle period so as to depressurize that adsorber to substantially the lower pressure while delivering the second product gas; and
(d) returning light reflux gas enriched in the less readily adsorbed component from the light reflux return ports so as to purge the adsorber in the latter part of the exhaust interval and then to partially repressurize the adsorber prior to the next feed interval,
so that feed gas is continuously supplied to substantially one adsorber at time, and exhaust gas is continuously removed from substantially one adsorber at a time.
According to another aspect of the invention there is provided a process for pressure swing adsorption separation of a feed gas mixture containing a more readily adsorbed component and a less readily adsorbed component, with the more readily adsorbed component being preferentially adsorbed from the feed gas mixture by an adsorbent material under increase of pressure, so as to separate from the feed gas mixture a heavy product gas enriched in the more readily adsorbed component and a light product gas enriched in the less readily adsorbed component, providing for the process a cooperating set of three adsorbers within a rotor and equally spaced by 120xc2x0 angular separation about the axis defined by rotation of the rotor relative to a stator, and rotating the rotor so as to generate within each adsorber cyclic variations of pressure and flow at a cyclic period defined by the frequency of rotation along a flow path contacting the adsorbent material between first and second ends of the adsorber, the cyclic variations of pressure extending between a higher pressure and a lower pressure of the process; rotating the rotor so that the first ends of the adsorbers successively communicate to feed and exhaust ports provided in a first valve surface between the rotor and the stator, and the second ends of the adsorbers successively communicate to a light product port, to first, second and third light reflux exit ports and to first, second and third light reflux return ports provided in a second valve surface between the rotor and the stator; the process including for each of the adsorbers in turn the following cyclical steps in sequence:
(a) supplying feed gas mixture at a feed pressure through the feed port to the adsorber over a feed interval which is substantially ⅓ of the cycle period so as to pressurize the adsorber to substantially the higher pressure, and then to deliver light product gas from the light product port at substantially the higher pressure less flow frictional pressure drops;
(b) withdrawing a first light reflux gas enriched in the less readily adsorbed component from the first light reflux exit port at about the end of the feed interval;
(c) withdrawing a second light reflux gas enriched in the less readily adsorbed component from the first light reflux exit port to depressurize that adsorber after the feed interval;
(d) withdrawing a third light reflux gas enriched in the less readily adsorbed component from the first light reflux exit port to further depressurize that adsorber;
(e) withdrawing second product gas at an exhaust pressure through the exhaust port from the adsorber over an exhaust interval which is substantially ⅓ of the cycle period so as to further depressurize that adsorber to substantially the lower pressure while delivering the second product gas;
(f) returning third light reflux gas from the third light reflux return port which is receiving that gas after pressure letdown from another adsorber (whose phase is leading by 120xc2x0), so as to purge the adsorber in the latter part of the exhaust interval;
(g) returning second light reflux gas from the second light reflux return port so as to partially repressurize the adsorber prior to the next feed interval;
(h) returning first light reflux gas from the first light reflux return port which is receiving that gas after pressure letdown from another adsorber (whose phase is lagging by 120xc2x0), so as to further repressurize the adsorber prior to the next feed interval; and
(i) cyclically repeating the above steps,
so that feed gas is continuously supplied to substantially one adsorber at a time, and exhaust gas is continuously removed from substantially one adsorber at a time.
According to a further aspect of the invention there is provided a process for pressure swing adsorption separation of a feed gas mixture containing a more readily adsorbed component and a less readily adsorbed component, with the more readily adsorbed component being preferentially adsorbed from the feed gas mixture by an adsorbent material under increase of pressure, so as to separate from the feed gas mixture a heavy product gas enriched in the more readily adsorbed component and a light product gas enriched in the less readily adsorbed component, providing for the process a cooperating set of three adsorbers, and generating within each adsorber cyclic variations of pressure and flow at a cyclic period defined by the frequency of rotation along a flow path contacting the adsorbent material between first and second ends of the adsorber and with the cyclic phase 120xc2x0 staggered for each adsorber, the cyclic variations of pressure extending between a higher pressure and a lower pressure of the process; the process including for each of the adsorbers in turn the following cyclical steps in sequence:
(a) supplying feed gas mixture to the first end of the adsorber over a feed interval which is substantially ⅓ of the cycle period so as to pressurize the adsorber to substantially the higher pressure, and then to deliver light product gas from the second end of the adsorber at substantially the higher pressure less flow frictional pressure drops,
(b) withdrawing a first light reflux gas enriched in the less readily adsorbed component from the second end of the adsorber at about the end of the feed interval;
(c) withdrawing a second light reflux gas enriched in the less readily adsorbed component from the second end of the adsorber to depressurize that adsorber after the feed interval, and delivering the second light reflux gas to a buffer chamber;
(d) withdrawing a third light reflux gas enriched in the less readily adsorbed component from the second end of the adsorber to further depressurize that adsorber;
(e) withdrawing second product gas at an exhaust pressure from the first end of the adsorber over an exhaust interval which is substantially ⅓ of the cycle period so as to further depressurize that adsorber to substantially the lower pressure while delivering the second product gas;
(f) supplying third light reflux gas from another adsorber (whose phase is leading by 120xc2x0) to the second end of the adsorber, so as to purge the adsorber during the latter part of the exhaust interval;
(g) supplying second light reflux gas from the buffer chamber to the second end of the adsorber, so as to partially repressurize the adsorber prior to the next feed interval;
(h) supplying third light reflux gas from another adsorber (whose phase is leading by 120xc2x0) to the second end of the adsorber, so as to further repressurize the adsorber prior to the next feed interval; and
(i) cyclically repeating the above steps,
while feed gas is continuously supplied to substantially one adsorber at a time, and exhaust gas is continuously removed from substantially one adsorber at a time.
According to another aspect of the invention there is provided an apparatus for pressure swing adsorption separation of a gas mixture containing a more readily adsorbed component and a less readily adsorbed component, with the more readily adsorbed component being preferentially adsorbed from the gas mixture by an adsorbent material under increase of pressure between a lower pressure and a higher pressure, so as to separate from the gas mixture a heavy product gas enriched in the more readily adsorbed component and a light product gas depleted in the more readily adsorbed component; the apparatus including an adsorber rotor cooperating with a stator mutually defining the rotational axis of the rotor and with rotor drive means to rotate the rotor at a rotational period which defines a pressure swing adsorption cycle period, the rotor containing a cooperating set of three adsorbers equally angularly spaced about the rotational axis, each adsorber having a flow path contacting the adsorbent material between first and second ends of the adsorber, the first ends of the adsorbers communicating by first apertures to a first valve surface between the rotor and the stator, and the second ends of the adsorbers communicating by second apertures to a second valve surface between the rotor and the stator; the first valve surface having feed and exhaust ports engaging successively in fluid communication with the first apertures, and the first valve surface having a light product port, and a first, second and third light reflux exit ports and first, second and third light reflux return ports engaging successively in fluid communication with the second apertures; the apparatus further including feed supply means communicating to the feed port and second product exhaust means communicating to the exhaust port; the first and third light reflux exit ports communicating directly to the first and third light reflux return ports respectively, and the second light reflux exit port communicating to a buffer chamber communicating in turn to the second light reflux return port; and the angular positions and widths of the ports and apertures being configured so that for each adsorber in sequence the following steps are performed:
(a) the first aperture of the adsorber is opened to the feed port through which feed gas mixture is supplied by the feed supply means over a feed interval of substantially ⅓ of the cycle period so as to pressurize the adsorber to substantially the higher pressure, while the second aperture of the adsorber is then opened to the light product port in the feed interval so as to deliver light product gas at substantially the higher pressure less flow frictional pressure drops;
(b) the second aperture of the adsorber is opened sequentially to the first, second and third light reflux exit ports so as to deliver light reflux gas enriched in the less readily adsorbed component and to depressurize the adsorber after the feed interval;
(c) the first aperture of the adsorber is opened to the exhaust port through which second product gas is exhausted by the second product exhaust means at an exhaust pressure over an exhaust interval which is substantially ⅓ of the cycle period so as to depressurize that adsorber to substantially the lower pressure and to deliver the second product gas;
(d) the second aperture of the adsorber is opened sequentially to the third, second and first light reflux return ports so as to purge the adsorber in the latter part of the exhaust interval and then to partially repressurize the adsorber prior to the next feed interval.
Further objects and advantages of the invention will become apparent from the description of preferred embodiments of the invention below.